Cast-iron insert and method of manufacturing same

ABSTRACT

A cylinder liner has an outer circumferential surface around which another metal is to be cast. The cylinder liner also has a plurality of protrusions disposed on the outer circumferential surface and having respective substantially conical undercuts or necks which are progressively spread outwardly from the outer circumferential surface. The protrusions have respective flat faces on the distal ends of the undercuts.

TECHNICAL FIELD

The present invention relates to a cast-iron insert over which anothermetal, e.g., aluminum, is to be cast, and a method of manufacturing sucha cast-iron insert.

BACKGROUND ART

For example, cylinder blocks for use in automotive engines are made ofan aluminum alloy for producing lighter engines. The cylinder blocksinclude cast-iron cylinder sleeves or liners (inserts) to providewear-resistant inner surfaces against which pistons slide back andforth. Brake drums for automobiles also use cast-iron shoes (inserts).

When a metal, e.g., an aluminum alloy, is to be cast around a cast-ironinsert, it is necessary that the cast-iron insert and the aluminum alloybe held in intimate contact with each other and that the aluminum alloyfill surface irregularities of the cast-iron insert. To meet suchrequirements, Japanese laid-open patent publication No. 2001-170755discloses a cast-iron insert having surface irregularities whose maximumheight ranges from 65 μm to 260 μm and whose average interval rangesfrom 0.6 mm to 1.5 mm.

According to the above publication, an aluminum alloy is cast around theouter peripheral surface of the cast-iron insert by a die-castingprocess to obtain a product where the aluminum alloy well fills thesurface irregularities of the outer peripheral surface of the cast-ironinsert and the cast-iron insert is held in highly intimate contact withthe aluminum alloy.

To form the desired outer surface of the cast-iron insert, there isemployed a facing material in the form of a suspension which contains amixture of 20 weight % to 45 weight % of silica sand having an averageparticle diameter in the range from 0.05 mm to 0.5 mm, 10 weight % to 30weight % of silica flour having an average particle diameter of 0.1 mmor less, 2 weight % to 10 weight % of a binder, and 30 weight % to 60weight % of water.

After the inner surface of a heated mold is coated with the above facingmaterial, the facing material is dried. When the facing material isdried, the facing material produces a vapor through holes therein,forming countless minute recesses in the inner surface of the mold. Whenmolten cast iron is then poured into the mold, the produced cast-ironinsert has an outer surface having spines corresponding to the recessesin the inner surface of the mold.

As shown in FIG. 9 of the accompanying drawings, a cast-iron insert 1has an outer surface 3 having needle-like spines 2. When an aluminumalloy 4 is cast around the outer surface 3 of the cast-iron insert 1, acast product 5 is produced. Since the outer surface 3 of the cast-ironinsert 1 has a plurality of spines 2, the cast aluminum alloy 4 isprevented from being relatively displaced with respect to the cast-ironinsert 1 in the directions indicated by the arrow A, and is subject toreduced residual stresses.

However, the cast-iron insert 1 peels off the aluminum alloy 4 in thedirections indicated by the arrow B parallel to the spines 2. When thecast-iron insert 1 peels off the aluminum alloy 4, the cast-iron insert1 is brought out of close contact with the aluminum alloy 4, and thearea of contact between the cast-iron insert 1 and the aluminum alloy 4is reduced, thus lowering the thermal conductivity of the cast product5.

After the cast-iron insert 1 is manufactured by casting, the innersurface (sliding surface) of the cast-iron insert 1 needs to bemachined. When the inner surface of the cast-iron insert 1 is machined,the outer surface 3 of the cast-iron insert 1 is clamped by a clampmechanism.

Because the spines 2 project from the outer surface 3 of the cast-ironinsert 1, the clamp mechanism has its clamping surface held inpoint-to-point contact with the tip ends of the spines 2. As a result,the area of contact between the clamping surface and the cast-ironinsert 1 is relatively small. On account of the relatively small area ofcontact between the clamping surface and the cast-iron insert 1, thecast-iron insert 1 is not positioned accurately while the inner surfaceof the cast-iron insert 1 is being machined. Consequently, the innersurface of the cast-iron insert 1 cannot be machined accurately.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a cast-iron insertwhich can be brought into increased intimate contact with another metaleffectively by a simple process and can be clamped in position with adesired level of accuracy.

Another object of the present invention is to provide a method ofmanufacturing a cast-iron insert which can be brought into increasedintimate contact with another metal effectively by a simple process andcan maintain a desired level of thermal conductivity.

According to the present invention, a cast-iron insert around whichanother metal is to be cast has a surface for contact with a molten massof the other metal to be cast around the cast-iron insert, and aplurality of protrusions disposed on the surface. The protrusions haverespective substantially conical undercuts or necks which areprogressively spread outwardly from the surface.

The substantially conical undercuts that progressively spread outwardlyfrom the surface of the cast-iron insert in various different directionsallow the cast-iron insert and the other metal, e.g., an aluminum alloy,cast therearound to be held in intimate contact with each other. Theprotrusions have a much larger surface area than the conventionalspines. When the cast-iron insert is actually used, the heat generatedin the cast-iron insert by another member which slides against thecast-iron insert can well be transmitted to the aluminum alloy.Accordingly, the cast-iron insert has a high heat radiation capability.

The protrusions have respective flat faces on the distal ends of theundercuts or necks which are progressively spread outwardly from thesurface of the cast-iron insert. Consequently, the area of contactbetween the outer circumferential surface of the cast-iron insert andthe clamping surface of a clamp mechanism which clamps the cast-ironinsert in position is much larger than the area of contact between theouter circumferential surface of the conventional spikes and theclamping surface. Stated otherwise, while the conventional spikes andthe clamping surface are held in point-to-point contact with each other,the cast-iron insert and the clamping surface are held in face-to-facecontact with each other. As a result, the cast-iron insert can beclamped in position with increased accuracy and hence can be machinedneatly with increased accuracy.

According to the present invention, a cast-iron insert is manufacturedby coating an inner surface of a mold with a facing material containinga thermally insulating material, a binder, a parting agent, a surfaceactive agent, and water, replacing an existing atmosphere in the moldwith an inactive gas atmosphere, and rotating the mold which has beencoated with the facing material and simultaneously pouring molten castiron into the mold, to produce a cast-iron insert having a surface forcontact with a molten mass of another metal to be cast around thecast-iron insert, and a plurality of protrusions disposed on the surfaceand having respective substantially conical undercuts or necks which areprogressively spread outwardly from the surface.

Specifically, when the inner surface of the mold is coated with thefacing material, part of the facing material swells outwardly into anumber of spherical bulges under surface tension because of the surfaceactive agent contained in the facing material. Therefore, the facingmaterial is provided with the spherical bulges, each with an undercut,projecting from a facing material surface over the inner surface of themold.

Then, the existing atmosphere in the mold is replaced with the inactivegas atmosphere. Therefore, no oxide film is formed on the surface of themolten cast iron as it is poured in the mold. As a result, the moltencast iron has its fluidity kept well in the mold. Consequently, themolten cast iron flows smoothly in the mold and reliably fills thespaces around the spherical bulges and the undercuts. When the cast ironis cooled into the cast-iron insert, it has its surface shapedaccurately complementarily to the surface configuration of the facingmaterial.

Thus, the cast-iron insert has the protrusions, each with thesubstantially conical undercut or neck progressively spread outwardly,firmly and neatly formed on the surface thereof. The protrusions arehighly effective to keep the cast-iron insert in intimate contact withthe aluminum alloy cast therearound, and also make the cast-iron inserthighly thermally conductive with respect to the aluminum alloy.

The facing material contains 20 weight % to 35 weight % of diatomaceousearth as the thermally insulating material, 1 weight % to 7 weight % ofbentonite as the binder, 1 weight % to 5 weight % of the parting agent,5 ppm to 50 ppm of the surface active agent, and the remainder of water.

If the diatomaceous earth were less than 20 weight %, then the facingmaterial would fail to be thermally insulative. If the diatomaceousearth were more than 35 weight %, then the facing material would have anincreased viscosity and would become less flowable than desired. If thebentonite were less than 1 weight %, then the facing material would loseits binding ability, allowing the other constituents thereof toseparate. If the bentonite were more than 7 weight %, then the facingmaterial would become too viscous to disintegrate after the cast-ironinsert has been cast to shape.

If the parting agent were less than 1 weigh %, then the facing materialwould lose its parting ability. If the parting agent were more than 5weight %, then water contained in the parting agent would be turned intoa gas due to the heat of the molten cast iron, producing blow holes inthe cast-iron insert.

If the surface active agent were less than 5 ppm, then it would fail tokeep the bulges spherical in shape. If the surface active agent weremore than 50 ppm, then the facing material would be foamed.

The mold is rotated at a mold G No. ranging from 25 G to 35 G when theinner surface of the mold is coated with the facing material. If themold G No. were less than 25 G, then the spherical bulges would not bedeformed sufficiently, resulting in an unduly wide interval betweenadjacent ones of the spherical bulges. The unduly widely spacedspherical bulges would fail to give desired undercuts to the protrusionsof the cast-iron insert, which would then not be able to adhere firmlyto the aluminum alloy. If the mold G No. were more than 35 G, then thespherical bulges would be deformed excessively, resulting in an undulynarrow interval between adjacent ones of the spherical bulges. Theunduly narrowly spaced spherical bulges would reduce the diameter of thenecks of the protrusions of the cast-iron insert, which would then beliable to be broken off.

The mold G No. is represented by (the centrifugal acceleration of themold/the gravitational acceleration). If the mold G No. is expressedusing the diameter D (cm) of the cylindrical mold and the rotationalspeed N (rpm) of the mold, then the mold G No. is equal to DN²/17900(see Japanese laid-open patent publication No. 2002-283025 for details).Therefore, the mold G No. can be obtained from the diameter D and therotational speed N.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an exploded perspective view of a cylinder block to be castaround a cylinder liner as a cast-iron insert according to an embodimentof the present invention;

FIG. 2 is a fragmentary perspective view of the cylinder liner, the viewshowing protrusions on the cylinder liner;

FIG. 3 is an enlarged fragmentary cross-sectional view of the cylinderblock;

FIG. 4 is an enlarged fragmentary cross-sectional view illustrative ofthe manner in which a mold is coated with a facing material;

FIG. 5 is an enlarged fragmentary cross-sectional view illustrative ofthe manner in which a molten metal is poured into the mold;

FIG. 6 is a fragmentary perspective view illustrative of the manner inwhich the cylinder liner is positioned by a clamp mechanism;

FIG. 7 is an enlarged fragmentary cross-sectional view of a facingmaterial applied at a low mold G No.;

FIG. 8 is an enlarged fragmentary cross-sectional view of a facingmaterial applied at a high mold G No.; and

FIG. 9 is an enlarged fragmentary cross-sectional view of a conventionalinsert.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 shows in exploded perspective a cylinder block 12 to be castaround a cylinder liner or sleeve 10 as a cast-iron insert according tothe present invention.

As shown in FIG. 1, the cylinder block 12 includes a block 14 made of analuminum alloy, for example, to produce lighter engines. The cylinderblock 12 also includes a plurality of cylinder liners or sleeves 10 (oneshown) around which an aluminum alloy is cast as the block 14.

Each of the cylinder liners 10 is molded of cast iron according to acentrifugal casting process. As shown in FIG. 2, the cylinder liner 10has a plurality of protrusions 20 disposed on an outer circumferentialsurface 16 thereof over which the aluminum alloy is to be cast. Each ofthe protrusions 20 has a substantially conical undercut or neck 18 whichis progressively spread outwardly and a flat outer face 21 on the distalend of the undercut or neck 18.

If the outer circumferential surface 16 of the cylinder liner 10 has adiameter ranging from 60 mm to 100 mm, then the height of eachprotrusion 20 from the outer circumferential surface 16 is in the rangefrom 0.5 mm to 1.2 mm. The cylinder liner 10 has an inner surface 10 aserving as a sliding surface against which a piston will slide back andforth. After the cylinder liner 10 has been cast to shape, the innersurface 10 a is machined.

As shown in FIG. 3, when the block 14 of the cylinder block 12 is castaround the cylinder liner 10, the aluminum alloy of the block 14 fillsup spaces between the protrusions 20 of the cylinder liner 10, thusforming spherical joints 22 on the block 14.

A process of manufacturing the cylinder liner (cast-iron insert) 10,i.e., a method of manufacturing the cast-iron insert according to thepresent invention, will be described below.

As shown in FIG. 4, a mold 30 of a centrifugal casting apparatus is of acylindrical shape and is rotatably supported by an actuator (not shown).

While the mold 30 is being rotated at a mold G No. ranging from 25 G to35 G, an inner circumferential surface 34 of the mold 30 is coated witha facing material 36. The facing material 36 contains a thermallyinsulating material, a binder, a parting agent, a surface active agent,and water. Specifically, the facing material 36 contains 20 weight % to35 weight % of diatomaceous earth as the thermally insulating material,1 weight % to 7 weight % of bentonite as the binder, 1 weight % to 5weight % of the parting agent, 5 ppm to 50 ppm of the surface activeagent, and the remainder of water.

The mold G No. is represented by (the centrifugal acceleration of themold 30/the gravitational acceleration). If the mold G No. is expressedusing the diameter D (cm) of the cylindrical mold 30 and the rotationalspeed N (rpm) of the mold 30, then the mold G No. is equal to DN²/17900(see Japanese laid-open patent publication No. 2002-283025 for details).Therefore, the mold G No. can be obtained from the diameter D and therotational speed N.

When the inner circumferential surface 34 of the mold 30 is coated withthe facing material 36, part of the facing material 36 swells outwardlyfrom an outer facing material surface 36 a under surface tension becauseof the surface active agent contained in the facing material 36, thusforming a number of spherical bulges 36 b on the outer facing materialsurface 36 a. Each of the bulges 36 b has an undercut 36 c.

Then, the atmosphere in the mold 30 is replaced with an inactive gasatmosphere containing an argon gas. Thereafter, as shown in FIG. 5,molten cast iron 40 is poured in the mold 30 while the mold 30 is beingrotated at a mold G No. ranging from 100 G to 135 G.

The molten cast iron 40 fills the mold 30, covering the spherical bulges36 b of the facing material 36. When the molten cast iron 40 issubsequently cooled, the molded cast iron has a surface complementary tothe outer facing material surface 36 a and the spherical bulges 36 bincluding the undercuts 36 c. In this manner, the cylindrical cylinderliner 10 having the outer circumferential surface 16 with theprotrusions 20 disposed thereon is formed in the mold 30.

In the present embodiment, the facing material 36 contains the thermallyinsulating material, the binder, the parting agent, the surface activeagent, and the water. The thermally insulating material comprisesdiatomaceous earth and has a function to keep the molten cast iron 40poured into the mold 30 at an optimum temperature. The diatomaceousearth is added in the range from 20 weight % to 35 weight %. If thediatomaceous earth were less than 20 weight %, then the facing material36 would fail to be thermally insulative. If the diatomaceous earth weremore than 35 weight %, then the facing material 36 would have anincreased viscosity and would become less flowable than desired.

The binder has a function to keep the bulges 36 b spherical in shape,and comprises bentonite, for example. The bentonite is added in therange from 1 weight % to 7 weight %. If the bentonite were less than 1weight %, then the facing material 36 would lose its binding ability,allowing the other constituents thereof to separate. If the bentonitewere more than 7 weight %, then the facing material 36 would become tooviscous to disintegrate after the cylinder liner 10 has been cast toshape.

The parting agent is added in the range from 1 weight % to 5 weight %.If the parting agent were less than 1 weigh %, then the facing material36 would lose its parting ability. If the parting agent were more than 5weight %, then water contained in the parting agent would be turned intoa gas due to the heat of the molten cast iron 40, producing blow holesin the cylinder liner 10.

The surface active agent has a function to increase the surface tensionof the facing material 36 to keep the bulges 36 b spherical in shape.The surface active agent is added in the range from 5 ppm to 50 ppm. Ifthe surface active agent were less than 5 ppm, then it would fail tokeep the bulges 36 b spherical in shape. If the surface active agentwere more than 50 ppm, then the facing material 36 would be foamed.

According to the present embodiment, after the inner circumferentialsurface 34 of the mold 30 has been coated with the facing material 36,the atmosphere in the mold 30 is replaced with an inactive gasatmosphere, and then the molten cast iron 40 is poured in the mold 30.Therefore, no oxide film is formed on the surface of the molten castiron 40 as it is poured in the mold 30. As a result, the molten castiron 40 has its fluidity kept well in the mold 30. Consequently, themolten cast iron 40 flows smoothly in the mold 30 and reliably fills thespaces around the spherical bulges 36 b and the undercuts 36 c. When thecast iron 40 is cooled into the cylinder liner 10, it has its surfaceshaped accurately complementarily to the surface configuration of thefacing material 36.

The cylinder liner 10 has the protrusions 20, each with thesubstantially conical undercut or neck 18 progressively spreadoutwardly, firmly and neatly formed on the outer circumferential surface16 thereof. The protrusions 20 are highly effective to keep the cylinderliner 10 in intimate contact with the block 14 cast therearound, andalso make the cylinder liner 10 highly thermally conductive with respectto the block 14.

As shown in FIG. 6, the cylinder liner 10 which has been cast to shapeis positioned and held by a clamp mechanism 50, and the inner surface 10a thereof is machined by a machine tool, not shown. While the innersurface 10 a of the cylinder liner 10 is being machined, the clampmechanism 50 has a clamping surface 52 held in face-to-face contact withsome of the flat faces 21 of the protrusions 20 of the cylinder liner10.

Since the clamping surface 52 a of the clamp mechanism 50 holds thecylinder liner 10 in face-to-face contact therewith, it provides a muchgreater area of contact with the cylinder liner 10 than it wouldotherwise hold the cylinder liner 10 in point-to-point contact with theconventional spines 2 (see FIG. 9). Accordingly, the clamp mechanism 50can clamp the cylinder liner 10 securely and accurately in position,allowing the inner surface 10 a thereof to be machined accurately.

After the cylinder liner 10 has been machined on the inner surface 10 athereof and otherwise machined, the cylinder liner 10 is placed in acylinder block casting mold, not shown. Then, another metal such as analuminum alloy, for example, is poured into the cylinder block castingmold, casting the block 14 around the cylinder liner 10. In this manner,the cylinder block 12 is manufactured.

According to the present embodiment, as shown in FIG. 2, the undercutsor necks 18 of the protrusions 20 are substantially conical in shape andare so shaped in both the circumferential direction (indicated by thearrow X) of the cylinder liner 10 and the axial direction (indicated bythe arrow Y) of the cylinder liner 10. Therefore, as shown in FIG. 3,the protrusions 20 of the cylinder liner 10 and the spherical joints 22on the block 14 are held in intimate contact with each other.

The cylinder liner 10 and the block 14 are prevented from beingdisplaced or shifted in the directions indicated by the arrow A, so thatresidual stresses produced in inter-bore regions 15 (see FIG. 1) of thecylinder block 12 can be reduced. The cylinder liner 10 and the block 14are also prevented from peeling off each other in the directionsindicated by the arrow B, so that the strength of intimate adhesionbetween the cylinder liner 10 and the block 14 is prevented from beingreduced.

Furthermore, the cylinder liner 10 and the block 14 are held in intimatecontact with each other through a large surface area. Accordingly, theheat generated in the cylinder liner 10 when the piston slides back andforth against the cylinder liner 10 can efficiently be transmitted tothe block 14, so that the cylinder block 12 has a high heat radiationcapability.

The mold G No. of the mold 30 is selected in the range from 25 G to 35 Gwhen the facing material 36 is applied to the mold 30. If the mold G No.were less than 25 G, then, as shown in FIG. 7, the spherical bulges 36 bwould not be deformed sufficiently, resulting in an unduly wide intervalH1 between adjacent ones of the spherical bulges 36 b. The unduly widelyspaced spherical bulges 36 b would fail to give desired undercuts 18 tothe protrusions 20 of the cylinder liner 10, which would then not beable to adhere firmly to the block 14.

If the mold G No. were more than 35 G, then, as shown in FIG. 8, thespherical bulges 36 b would be deformed excessively, resulting in anunduly narrow interval H2 between adjacent ones of the spherical bulges36 b. The unduly narrowly spaced spherical bulges 36 b would reduce thediameter of the necks 18 of the protrusions 20 of the cylinder liner 10,which would then be liable to be broken off.

In the present embodiment, the height of each protrusion 20 from theouter circumferential surface 16 is in the range from 0.5 mm to 1.2 mm.If the height of each protrusion 20 were less than 0.5 mm, then it wouldbe difficult to produce the undercuts or necks 18 of desired shape,which would then not be able to adhere firmly to the block 14. If theheight of each protrusion 20 were more than 1.2 mm, then the necks 18 ofthe protrusions 20 would undesirably be elongated and might possibly bebroken off.

In the present embodiment, the cylinder liner 10 has been described as acast-iron insert according to the present invention. However, thepresent invention is also applicable to a brake shoe for brake drums,for example, as a cast-iron insert.

If a brake shoe has an outer dimension of about 130 mm, then protrusionson the brake shoe should preferably have a height in the range from 0.5mm to 2 mm.

INDUSTRIAL APPLICABILITY

According to the present invention, a cast-iron insert has a pluralityof protrusions disposed on the surface. The protrusions have respectivesubstantially conical undercuts or necks which are progressively spreadoutwardly from the surface in various different directions. Thesubstantially conical undercuts allow the cast-iron insert and the othermetal, e.g., an aluminum alloy, cast therearound to be held in intimatecontact with each other. The protrusions have a much larger surface areathan the conventional spines. When the cast-iron insert is actuallyused, the heat generated in the cast-iron insert can well be transmittedto the aluminum alloy. Accordingly, the cast-iron insert has a high heatradiation capability.

The protrusions have respective flat faces on the distal ends of theundercuts or necks which are progressively spread outwardly from thesurface of the cast-iron insert. Consequently, the area of contactbetween the outer circumferential surface of the cast-iron insert andthe clamping surface of a clamp mechanism which clamps the cast-ironinsert in position is much larger than the area of contact between theouter circumferential surface of the conventional spikes and theclamping surface. As a result, the cast-iron insert can be clamped inposition with increased accuracy and hence can be machined neatly withincreased accuracy.

According to the present invention, a cast-iron insert is manufacturedso that the protrusions are firmly formed on the surface of thecast-iron insert by a simple process. Each of the protrusions has asubstantially conical undercut or neck, and the undercut has a sphericalcontact portion. The protrusions are highly effective to keep thecast-iron insert in intimate contact with the aluminum alloy or the likecast therearound, and also make the cast-iron insert highly thermallyconductive with respect to the aluminum alloy or the like.

1. A cast-iron insert around which another metal is to be cast,comprising: a surface for contact with a molten mass of said other metalto be cast around the cast-iron insert; and a plurality of protrusionsdisposed on said surface and having respective substantially conicalundercuts which are progressively spread outwardly from said surface,wherein distal ends of said protrusions have respective flat faces, theflat faces allowing the cast-iron insert to be securely held inface-to-face contact by a clamping mechanism, wherein said undercutshave respective spherical contact portions, and said other metal is castaround said spherical contact portions, and wherein said cast-ironinsert comprises a cylinder liner.
 2. The cast-iron insert around whichanother metal is to be cast according to claim 1, wherein theprotrusions have a height in a range of 0.5 mm to 2 mm above the surfacefor contact.
 3. The cast-iron insert around which another metal is to becast according to claim 1, wherein the protrusions have a height in arange of 0.5 mm to 1.2 mm above the surface for contact.
 4. Thecast-iron insert around which another metal is to be cast according toclaim 1, wherein the surface of contact has a diameter ranging from 60mm to 100 mm.
 5. A method of manufacturing a cast-iron insert,comprising the steps of: coating an inner surface of a mold with afacing material containing a thermally insulating material, a binder, aparting agent, a surface active agent, and water; replacing an existingatmosphere in said mold with an inactive gas atmosphere; rotating saidmold which has been coated with said facing material and simultaneouslypouring molten cast iron into said mold, to produce a cast-iron inserthaving a surface for contact with a molten mass of another metal to becast around the cast-iron insert, and a plurality of protrusionsdisposed on said surface and having respective substantially conicalundercuts which are progressively spread outwardly from said surface;and positioning a clamping mechanism against respective flat facesformed at distal ends of said protrusions in order to securely hold thecast-iron insert, wherein said undercuts have respective sphericalcontact portions, and said other metal is cast around said sphericalcontact portions, and wherein said facing material contains 20 weight %to 35 weight % of diatomaceous earth as said thermally insulatingmaterial, 1 weight % to 7 weight % of bentonite as said binder, 1 weight% to 5 weight % of said parting agent, 5 ppm to 50 ppm of said surfaceactive agent, and the remainder of water.
 6. The method according toclaim 5, wherein said mold is rotated at a mold G No. ranging from 25 Gto 35 G when the inner surface of the mold is coated with the facingmaterial.
 7. The method according to claim 5, further comprising thestep of machining an inner surface of the cast-iron insert afterperforming the step of positioning the clamping mechanism against therespective flat faces at distal ends of said protrusions in order tosecurely hold the cast-iron insert.
 8. The method according to claim 5,wherein the protrusions have a height in a range of 0.5 mm to 2 mm abovethe surface for contact.
 9. The method according to claim 5, wherein theprotrusions have a height in a range of 0.5 mm to 1.2 mm above thesurface for contact.
 10. The method according to claim 5, wherein thesurface of contact has a diameter ranging from 60 mm to 100 mm.